Initial Allocations Between Debt and Equity Components of Debentures (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Face Amount Outstanding	$ 5,457
Debt Carrying Value	3,569	4,543
Exchangeable 2009 Senior Debentures 2.5% Due October 2029
Debt Instrument [Line Items]
Face Amount Outstanding	400
Debt Carrying Value	356	342
Exchangeable 2004 Senior Debentures 3.25% Due April 2024
Debt Instrument [Line Items]
Face Amount Outstanding	175
Debt Carrying Value	175	175
Exchangeable Senior Debentures
Debt Instrument [Line Items]
Initial Face Amount	900
Initial Debt Value	729
Initial Equity Value	158
Face Amount Outstanding	575
Debt Carrying Value	531
Unamortized Discount	44
Exchangeable Senior Debentures | Exchangeable 2009 Senior Debentures 2.5% Due October 2029
Debt Instrument [Line Items]
Initial Face Amount	400
Initial Debt Value	316
Initial Equity Value	82
Face Amount Outstanding	400
Debt Carrying Value	356
Unamortized Discount	44
Exchangeable Senior Debentures | Exchangeable 2004 Senior Debentures 3.25% Due April 2024
Debt Instrument [Line Items]
Initial Face Amount	500
Initial Debt Value	413
Initial Equity Value	76
Face Amount Outstanding	175
Debt Carrying Value	$ 175